The art of inkjet technology is relatively well-developed. Commercial products of recording or printing apparatus such as computer printers, graphics plotters, and facsimile machines employ inkjet technology for producing recorded media. Hewlett-Packard's contributions to this technology, ink-jet in particular, are described in various articles in the Hewlett-Packard Journal, Vol. 36, No. 5 (May 1985), Vol. 39, No. 4 (August 1988), Vol. 39, No. 5 (October 1988), Vol. 43, No. 4 (August 1992), Vol. 43, No. 6 (December 1992), and Vol. 45, No. 1 (February 1994).
A ink-jet image is formed when drops are ejected from a drop-generating device known as a "printhead" to form precise patterns on a recording medium such as paper, vellum, or acrylic slide material to name a few. The drop-generating device uses any suitable technology for selectively depositing ink on media such as thermal ink-jet or piezo to name a couple. In the case of thermal ink jet, a typical ink-jet printhead has an array of precisely formed nozzles attached to a thermal ink-jet printhead substrate. This substrate incorporates an array of ink ejection chambers that receive liquid fluid, such as ink, from a fluid reservoir in a print cartridge containing the printhead. Each ink ejection chamber in the printhead has a thin-film resistor, known as a "firing resistor," located opposite each nozzle so fluid can collect between the firing resistor and the nozzle. When the firing resistor is selectively activated, a small volume of fluid adjacent the firing resistor is heated, vaporizing a bubble of fluid, and thereby ejecting a drop of fluid from the printhead. The droplets strike the recording medium and then dry to form "dots" that, when viewed together, form the recorded image.
In general, the fluid in the fluid reservoir within the print cartridge has an operating pressure chosen with at least two limiting conditions. First, the operating pressure must be sufficiently negative, creating a "back-pressure", so that during printhead operation fluid does not run freely through the ink ejection chambers and exit from the nozzles. This phenomenon of free running fluid is called "drooling". Secondly, the operating pressure of the printhead must not be too negative so that when the firing resistor is heated, the vaporized bubble of fluid can overcome this operating back-pressure and eject a droplet of fluid from the ink ejection chamber. Most printheads today operate in a slight vacuum, typically in a gauge pressure range of between about -2 inches (minus two inches) of water to about -10 inches (minus ten inches) of water. Gauge pressure is pressure measured relative to atmospheric pressure outside of the print cartridge. Atmospheric pressure outside of the print cartridge is defined as 0 (zero) inches of water.
Some ink-jet printheads are located in printers or other media-recording apparatus having pressurized fluid supplies. Pressurized fluid systems enable fluid to be supplied to the printhead at higher fluid flow rates than non-pressurized systems, thus allowing for greater reliability and high print rate printing for applications such as large format or high density printing. The fluid in typical pressurized systems is pressurized from a fluid source to a supply pressure of between about +30 inches (plus thirty inches) of water to about +3 inches and is delivered to the printhead using either a tube or a conduit. A back-pressure regulator is normally located near the printhead, such as in a print cartridge containing the printhead, to reduce the supply pressure of the fluid down to the operating pressure required of the printhead.
Consumers, particularly of digital photography, are demanding fast printing speeds and photographic film quality results. To meet these consumer demands, as well as others, requires substantially increasing the rate of fluid ejected from the printhead. Another problem encountered when printing photographs onto recording medium at high speed is that the fluid leaving the printhead causes the back-pressure within the reservoir of the print cartridge to change, sometimes abruptly. Consistent drop volume for the fluid ejected is required for photographic quality, however, the drop volume is affected by the changing back-pressure. Printing at these high use rates requires that the regulator have a faster response time than required with low use rates to maintain adequate back-pressure regulation. If the back-pressure regulator cannot provide new fluid fast enough, the pressure will drop sufficiently low that the fluid ejected from the printhead will either cease or the quality of the drop will diminish. Conversely, if the flow of fluid into the reservoir from the back-pressure regulator is too great, the ability of the back-pressure regulator to stabilize sufficient back-pressure is affected when only low volumes of fluid are ejected from the printhead. It is essential that the drop volume of the fluid ejected from the printhead be consistent to achieve high print quality. Achieving consistent drop volume requires that the back-pressure range be controlled to an ever finer levels.
Another requirement for an improved back-pressure regulation is to accommodate air that is built up over time within the print cartridge reservoir. This air is introduced by diffusion through system components or tubing, at fluid interconnects in the pressurized system, or from air that has been released from the fluid itself through out-gassing. A pressurized system can introduce air either during refilling or replacement of the main fluid source. This air can also be released from the fluid either during changes in temperature or atmospheric pressure changes due to weather or elevation. Size constraints on the print cartridge often provide a limited capacity for warehousing air within a reservoir of fluid within the print cartridge. If the amount of air within the reservoir of the print cartridge becomes too large, either the print cartridge will not be able to supply a sufficient amount of ink during high speed, high density printing, or it may not allow the back-pressure regulator to operate properly. In addition, large amounts of air will respond to changes in atmospheric pressure and/or temperature. These responses may cause the printhead to drool (the air expanding) or to deprime (the air contracting). Depriming occurs when the ink within the printhead is drawn back into the reservoir. Therefore air within the reservoir causes the printing system to have a reduction in visual quality or to simply fail to work properly.